She (as a 16 year old girl) retired because she got tired of training and competing and also supposedly because "boys don't date girl with green hair" (chlorine effect on hair) see: http://www.smh.com.au/sport/swimming/the-race-is-never-over-for-olympic-superfish-20120321-1vkec.html

Shane Gould... only swimmer, male or female, to hold every single world freestyle swimming record from 100m to 1500m (achieved this at the age of 15); no track and field athlete could do or ever has done the same. Also won 3 Olympic gold, 1 silver and 1 bronze again at age 15 in 1972 (Munich games) and then promptly retired from competitive swimming age 16.